Protein A is a 41 kD cell wall protein from Staphylococcus aureas that binds with a high affinity to the Fc region of antibodies. For example, Protein A binds with high affinity to human IgG1, IgG2 and IgG4 as well as to mouse IgG2a and IgG2b. Protein A binds with moderate affinity to human IgM, IgA and IgE as well as to mouse IgG3 and IgG1.
Protein G is an immunoglobulin-binding protein expressed in group C and G Streptococcal bacteria. Protein G binds to a broader range of IgG subclasses than protein A. Protein L was first isolated from the surface of bacterial species Peptostreptococcus magnus and also binds immunoglobulins via interaction with the kappa light chain.
Protein A/G is a recombinant fusion protein that combines IgG binding domains of both Protein A and Protein G. Protein A/G contains four Fc binding domains from Protein A and two from Protein G.
Each of the above-described proteins have been described for purifying immunoglobulins, with a Protein A being most commonly described in antibody purification. Very generally, Protein A, Protein G, Protein L, or Protein A/G are used in affinity chromatography to bind target antibodies from samples. The antibodies (bound to Protein A, G, L, or A/G) can then be separated from most other components of sample, and optionally further purified using other purification steps.